Logs such as a connection cache entry timed out, Connection has been dropped are very common for TCP connections. This is due to inactivity of the session. The default TCP timeout setting for a rule is 15 minutes.
You can extend the timeout setting on specific rules for applications that experience long idle times, such as Citrix or MS Terminal Server.
Login to the SonicWall management interface.
Go to MANAGE | Rules | Access Rules, edit the appropriate rule by clicking the edit icon.
Click on Advanced tab.
If you would like for the access rule to timeout after a period of TCP inactivity, set the amount of time, in minutes, in the TCP Connection Inactivity Timeout (minutes) field. The default value is 15 minutes. If you would like for the access rule to timeout after a period of UDP inactivity, set the amount of time, in minutes, in the UDP Connection Inactivity Timeout (seconds) field. The default value is 30 seconds.
Click on OK button to update the rule.
There is a global TCP/UDP timeout setting available under MANAGE | Firewall Settings | Flood protection. There are separate tabs to set this for TCP and UDP.
CAUTION: Making changes globally is not recommended as this affects all access rules and will apply to all TCP/UDP connections on the firewall. This might result in stale connections being held up causing Core 0 Spikes.